A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In device manufacturing methods using lithographic apparatus, overlay is an important factor in the yield, i.e. the percentage of correctly manufactured devices. Overlay is the accuracy within which layers are printed in relation to layers that have previously been formed. The overlay error budget will often be 10 nm or less, and to achieve such accuracy, the substrate must be aligned to the reticle pattern (and therefore to the reticle itself) to be transferred with great accuracy. Typically an IC has several tens of layers, and reticle alignment (aligning the reticle pattern with the wafer or wafer stage) should be performed for each layer of each substrate, so that the image of the new layer is correctly aligned with the previous images/layers. Any distortions, deformations or any other alignment errors can have a negative impact on overlay.
This reticle alignment is performed by projecting the radiation beam onto a grating on the reticle. The resultant radiation beams emitted by the plurality of openings in the grating pass through the projection lens system of the lithographic apparatus, such that an image of the grating is produced on a photosensitive device, which itself has already been (or will be) accurately aligned in relation to the substrate. The light intensity detected by the photosensitive device is dependent on the relative position of the grating (and therefore the reticle), relative to the photosensitive device (and therefore the substrate), such that a detected light intensity maximum indicates that the reticle and substrate are properly aligned. Alternatively, or in addition, methods may be used which detect light minima to indicate proper alignment in combination with inversed alignment marks on the reticle. An image of the projected grating as seen with the photosensitive device is referred to as an “aerial image”, and extends in three dimensions.
In order to find the aligned position, a horizontal/vertical scan is performed in which, at each level of a defined number of z-levels, a move in the x-y plane is performed, centered approximately around the expected aligned position. The scan is performed as a continuous single scan back and forth over each z-level. A number of discrete samples are taken at sampling points along the scan path, either as a result of the radiation beam comprising a pulsed laser, or of the sampling being performed at discrete moments in time for continuous light sources.
Current state of the art is to measure the area in the vicinity of the aerial image in a linear manner with a discrete amount of z levels scanned in a linear manner. An issue with this type of scan is that all the relevant samples near the aligned position are measured within a short time period, which means that the measurement points are partly correlated, certainly for the low frequency range. Most of the noise impacting reticle alignment is at these low frequencies, resultant from low frequency disturbances in, for example, the liquid lens and the air along the optical path. Such disturbances might occur, for example, when different air and or water flows, each having different temperatures and or chemical components, are mixed.
A further issue with the type of scan described above is that the sampling is designed in such way that there are a number of particular frequencies in the higher frequency range at which the image sensor has a high sensitivity to measurement position noise. If a strong noise contribution is accidentally present at these frequencies, the image sensor performance during reticle alignment is significantly impaired.